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data-v-06225672><li data-v-06225672><a href="/" title="首页" class="iconfont icon-home router-link-active" data-v-06225672></a></li> <li data-v-06225672><a href="/mianshi/#面试" data-v-06225672>面试</a></li><li data-v-06225672><a href="/mianshi/#前端面试" data-v-06225672>前端面试</a></li><li data-v-06225672><a href="/mianshi/#进阶篇" data-v-06225672>进阶篇</a></li></ul> <div class="info" data-v-06225672><div title="作者" class="author iconfont icon-touxiang" data-v-06225672><a href="https://github.com/linxin1123" target="_blank" title="作者" class="beLink" data-v-06225672>lingxin</a></div> <div title="创建时间" class="date iconfont icon-riqi" data-v-06225672><a href="javascript:;" data-v-06225672>2023-02-26</a></div> <!----></div></div></div> <!----> <div class="content-wrapper"><div class="right-menu-wrapper"><div class="right-menu-margin"><div class="right-menu-title">目录</div> <div class="right-menu-content"></div></div></div> <h1><img src="">JavaScript进阶(一)<!----></h1>  <div class="theme-vdoing-content content__default"><h1 id="javascript进阶-一"><a href="#javascript进阶-一" class="header-anchor">#</a> JavaScript进阶(一)</h1> <h2 id="题目目录"><a href="#题目目录" class="header-anchor">#</a> 题目目录</h2> <ol><li><a href="#q1">bind、call、apply的区别</a></li> <li><a href="#q2">实现一个bind函数</a></li> <li><a href="#q3">实现一个call函数</a></li> <li><a href="#q4">实现一个apply函数</a></li> <li><a href="#q5">说下原型链</a></li> <li><a href="#q6">怎么判断对象类型</a></li> <li><a href="#q7">箭头函数的特点</a></li> <li><a href="#q8">this</a></li> <li><a href="#q9">async,await的优缺点</a></li> <li><a href="#q10">generator原理</a></li> <li><a href="#q11">Promise</a></li> <li><a href="#q12">如何实现一个promise</a></li> <li><a href="#q13"><code>==</code>和<code>===</code>的区别，什么情况用<code>==</code></a></li> <li><a href="#q14">基本数据类型和引用类型在存储上的差别</a></li> <li><a href="#q15">浏览器eventloop和Node中的有什么区别</a></li> <li><a href="#q16">setTimeout倒计时误差</a></li> <li><a href="#q17">数组降维</a></li> <li><a href="#q18">深拷贝</a></li> <li><a href="#q19">typeof 与instanceof的区别</a></li></ol> <h3 id="q1">1. bind、call、apply的区别</h3> <ul><li>call和apply都可以改变this的指向，传入参数的方式不同</li> <li>除了第一个参数外，call可以接收一个参数列表，apply只接收一个参数数组</li></ul> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">let</span> a<span class="token operator">=</span><span class="token punctuation">{</span>
    <span class="token literal-property property">value</span><span class="token operator">:</span><span class="token number">1</span>
<span class="token punctuation">}</span>

<span class="token keyword">function</span> <span class="token function">getValue</span><span class="token punctuation">(</span><span class="token parameter">name<span class="token punctuation">,</span>age</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
    
    console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>name<span class="token punctuation">)</span>
 console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>age<span class="token punctuation">)</span>
 console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">.</span>value<span class="token punctuation">)</span>

<span class="token punctuation">}</span>

<span class="token function">getValue</span><span class="token punctuation">.</span><span class="token function">call</span><span class="token punctuation">(</span>a<span class="token punctuation">,</span><span class="token string">'yck'</span><span class="token punctuation">,</span><span class="token string">'23'</span><span class="token punctuation">)</span>
<span class="token function">getValue</span><span class="token punctuation">.</span><span class="token function">apply</span><span class="token punctuation">(</span>a<span class="token punctuation">,</span><span class="token punctuation">[</span><span class="token string">'yck'</span><span class="token punctuation">,</span><span class="token string">'24'</span><span class="token punctuation">]</span><span class="token punctuation">)</span>

<span class="token comment">// 执行结果</span>
yck
<span class="token number">23</span> 
<span class="token number">1</span>  
yck
<span class="token number">24</span> 
<span class="token number">1</span>  
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br><span class="line-number">20</span><br><span class="line-number">21</span><br><span class="line-number">22</span><br></div></div><blockquote><p>bind和其他两个方法作用也是一致的，只是该方法会返回一个函数，并且我们可以通过bind实现柯里化</p></blockquote> <h3 id="q2">2. 实现一个bind函数</h3> <ul><li>对于实现以下几个函数，可以从几个方面思考
<ul><li>不传入参数，默认this为window</li> <li>改变this指向，新的对象执行该函数，思路为新的对象添加一个函数，执行完后删除</li></ul></li></ul> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token class-name">Function</span><span class="token punctuation">.</span>prototype<span class="token punctuation">.</span><span class="token function-variable function">myBind</span><span class="token operator">=</span><span class="token keyword">function</span><span class="token punctuation">(</span><span class="token parameter">context</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
    
    <span class="token keyword">if</span><span class="token punctuation">(</span><span class="token keyword">typeof</span> <span class="token keyword">this</span> <span class="token operator">!==</span><span class="token string">'function'</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
        <span class="token keyword">throw</span> <span class="token keyword">new</span> <span class="token class-name">TypeError</span><span class="token punctuation">(</span><span class="token string">'Error'</span><span class="token punctuation">)</span>
    <span class="token punctuation">}</span>
    
   <span class="token keyword">var</span> _this<span class="token operator">=</span><span class="token keyword">this</span>
   
   <span class="token keyword">var</span> args<span class="token operator">=</span><span class="token punctuation">[</span><span class="token operator">...</span>arguments<span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">slice</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span>
   
   <span class="token comment">// 返回一个函数</span>
   
   <span class="token keyword">return</span> <span class="token keyword">function</span> <span class="token constant">F</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
       
       <span class="token keyword">if</span><span class="token punctuation">(</span><span class="token keyword">this</span> <span class="token keyword">instanceof</span> <span class="token class-name">F</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
           
           <span class="token keyword">return</span> <span class="token keyword">new</span> <span class="token class-name">_this</span><span class="token punctuation">(</span><span class="token operator">...</span>args<span class="token punctuation">,</span><span class="token operator">...</span>arguments<span class="token punctuation">)</span>
       <span class="token punctuation">}</span>
       
       <span class="token keyword">return</span> <span class="token function">_this</span><span class="token punctuation">.</span><span class="token function">apply</span><span class="token punctuation">(</span>context<span class="token punctuation">,</span>args<span class="token punctuation">.</span><span class="token function">concat</span><span class="token punctuation">(</span><span class="token operator">...</span>arguments<span class="token punctuation">)</span><span class="token punctuation">)</span>
   <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br><span class="line-number">20</span><br><span class="line-number">21</span><br><span class="line-number">22</span><br></div></div><h3 id="q3">3. 实现一个call函数</h3> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token class-name">Function</span><span class="token punctuation">.</span>prototype<span class="token punctuation">.</span><span class="token function-variable function">myCall</span><span class="token operator">=</span><span class="token keyword">function</span><span class="token punctuation">(</span><span class="token parameter">context</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
	<span class="token keyword">var</span> context<span class="token operator">=</span>context<span class="token operator">||</span>window
    
    <span class="token comment">// 给context添加一个属性</span>
    <span class="token comment">// getValue.call(a,'yck','23')=&gt;a.fn=getValue</span>
    
    context<span class="token punctuation">.</span>fn<span class="token operator">=</span><span class="token keyword">this</span>
    
    <span class="token keyword">var</span> args<span class="token operator">=</span><span class="token punctuation">[</span><span class="token operator">...</span>arguments<span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">slice</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span>
    
    
     <span class="token comment">// getValue.call(a, 'yck', '24') =&gt; a.fn('yck', '24')</span>
 <span class="token keyword">var</span> result <span class="token operator">=</span> context<span class="token punctuation">.</span><span class="token function">fn</span><span class="token punctuation">(</span><span class="token operator">...</span>args<span class="token punctuation">)</span>
 <span class="token comment">// 删除 fn</span>
 <span class="token keyword">delete</span> context<span class="token punctuation">.</span>fn
 <span class="token keyword">return</span> result
    
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br></div></div><h3 id="q4">4. 实现一个apply函数</h3> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token class-name">Function</span><span class="token punctuation">.</span>prototype<span class="token punctuation">.</span><span class="token function-variable function">myApply</span><span class="token operator">=</span><span class="token keyword">function</span><span class="token punctuation">(</span><span class="token parameter">context</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
    
    <span class="token keyword">var</span> context<span class="token operator">=</span>context<span class="token operator">||</span>window
    <span class="token comment">// this为函数的直接调用者,也就是getValue.myApply(a,['yck','24'])的getValue</span>
    
    <span class="token comment">// context为 a</span>
    context<span class="token punctuation">.</span>fn<span class="token operator">=</span><span class="token keyword">this</span>
    
    <span class="token comment">// 数组传参（a,['yck','24']）</span>
    
    <span class="token keyword">var</span> result<span class="token punctuation">;</span>
    
    <span class="token keyword">if</span><span class="token punctuation">(</span>arguments<span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
        result<span class="token operator">=</span>context<span class="token punctuation">.</span><span class="token function">fn</span><span class="token punctuation">(</span><span class="token operator">...</span>arguments<span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">]</span><span class="token punctuation">)</span>
    <span class="token punctuation">}</span><span class="token keyword">else</span><span class="token punctuation">{</span>
        result<span class="token operator">=</span>context<span class="token punctuation">.</span><span class="token function">fn</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
    <span class="token punctuation">}</span>
    
    
    <span class="token keyword">delete</span> context<span class="token punctuation">.</span>fn
 <span class="token keyword">return</span> result
    
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br><span class="line-number">20</span><br><span class="line-number">21</span><br><span class="line-number">22</span><br><span class="line-number">23</span><br></div></div><h3 id="q5">5. 简单说下原型链</h3> <ul><li>每个函数都有prototype属性，除了Function.prototype.bind(),该属性指向原型</li> <li>每个对象都有<code>__proto__</code>属性,指向了创建该对象的构造函数的原型，其实这个属性指向了[[prototype]],但是[[prototype]]是内部属性,我们并不能访问到，所有使用<code>__proto__</code>来访问</li> <li>对象可以通过<code>__proto__</code>来寻找不属于该对象的属性，<code>__proto__</code>将对象连接起来组成了原型链</li></ul> <h3 id="q6">6. 怎么判断对象类型</h3> <ul><li>可以通过Object.prototype.toString.call(xx),这样我们就可以获得类似[object Type]的字符串</li> <li>instanceof 可以正确的判断对象的类型，因为内部机制是通过判断对象的原型链中是不是能找到类型的prototype</li></ul> <h3 id="q7">7. 箭头函数的特点</h3> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">function</span> <span class="token function">a</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
    
    <span class="token keyword">return</span> <span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token operator">=&gt;</span><span class="token punctuation">{</span>
        <span class="token keyword">return</span> <span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token operator">=&gt;</span><span class="token punctuation">{</span>
            console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">)</span>
        <span class="token punctuation">}</span>
    <span class="token punctuation">}</span>
<span class="token punctuation">}</span>

console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token function">a</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br></div></div><blockquote><p>箭头函数其实是没有this的，这个函数中的this只取决于他外面的第一个不是箭头函数的函数的this，在这个例子中，因为调用a符合前面代码的第一个情况，所有this是window，并且this一旦绑定了上下文，就不会被任何代码改变</p></blockquote> <h3 id="q8">8. This</h3> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">function</span> <span class="token function">foo</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">.</span>a<span class="token punctuation">)</span>
<span class="token punctuation">}</span>
<span class="token keyword">var</span> a <span class="token operator">=</span> <span class="token number">1</span>
<span class="token function">foo</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
<span class="token keyword">var</span> obj <span class="token operator">=</span> <span class="token punctuation">{</span>
<span class="token literal-property property">a</span><span class="token operator">:</span> <span class="token number">2</span><span class="token punctuation">,</span>
<span class="token literal-property property">foo</span><span class="token operator">:</span> foo
<span class="token punctuation">}</span>
obj<span class="token punctuation">.</span><span class="token function">foo</span><span class="token punctuation">(</span><span class="token punctuation">)</span>

<span class="token comment">// 以下情况是优先级最⾼的，`this` 只会绑定在 `c` 上，不会被任何⽅式修改 `this` 指向</span>
<span class="token keyword">var</span> c <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">foo</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
c<span class="token punctuation">.</span>a <span class="token operator">=</span> <span class="token number">3</span>
console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>c<span class="token punctuation">.</span>a<span class="token punctuation">)</span>

<span class="token comment">// 执行结果</span>
<span class="token keyword">undefined</span>
<span class="token number">2</span>

<span class="token keyword">undefined</span>
<span class="token number">3</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br><span class="line-number">20</span><br><span class="line-number">21</span><br><span class="line-number">22</span><br></div></div><h3 id="q9">9. async、await的优缺点</h3> <blockquote><p>async和await相比直接使用Promise来说，优势在于处理then的调用链，能够更清晰准确的写出代码，缺点在于滥用await可能会导致性能问题，因为await会阻塞代码，也许之后的异步代码并不依赖于前者，当任然需要等待前者，导致代码失去了并发性</p></blockquote> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">var</span> a <span class="token operator">=</span> <span class="token number">0</span>
<span class="token keyword">var</span> <span class="token function-variable function">b</span> <span class="token operator">=</span> <span class="token keyword">async</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
 a <span class="token operator">=</span> a <span class="token operator">+</span> <span class="token keyword">await</span> <span class="token number">10</span>
 console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'2'</span><span class="token punctuation">,</span> a<span class="token punctuation">)</span> <span class="token comment">// -&gt; '2' 10</span>
 a <span class="token operator">=</span> <span class="token punctuation">(</span><span class="token keyword">await</span> <span class="token number">10</span><span class="token punctuation">)</span> <span class="token operator">+</span> a
 console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'3'</span><span class="token punctuation">,</span> a<span class="token punctuation">)</span> <span class="token comment">// -&gt; '3' 20</span>
<span class="token punctuation">}</span>
<span class="token function">b</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
a<span class="token operator">++</span>
console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'1'</span><span class="token punctuation">,</span> a<span class="token punctuation">)</span> <span class="token comment">// -&gt; '1' 1</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br></div></div><ul><li>b被当成为异步函数，会在之后被执行，同步任务先被执行</li></ul> <h3 id="q10">10. generator原理</h3> <blockquote><p>Generator是es6中新增的语法，和Promise一样，异步编程</p></blockquote> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token comment">// 使用*代表这是一个Generator函数</span>
<span class="token comment">// 内部可以通过yield暂停代码</span>
<span class="token comment">// 通过调用next恢复执行</span>

<span class="token keyword">function</span><span class="token operator">*</span> <span class="token function">test</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
    <span class="token keyword">let</span> a<span class="token operator">=</span><span class="token number">1</span><span class="token operator">+</span><span class="token number">2</span>
    <span class="token keyword">yield</span> <span class="token number">2</span><span class="token punctuation">;</span>
    <span class="token keyword">yield</span> <span class="token number">3</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token keyword">let</span> b<span class="token operator">=</span><span class="token function">test</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>b<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// {value:2,done:false}</span>

console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>b<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span> 
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br></div></div><ul><li>Genarator的简单实现</li></ul> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">function</span> <span class="token function">generator</span><span class="token punctuation">(</span><span class="token parameter">cb</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
    <span class="token keyword">return</span> <span class="token punctuation">(</span><span class="token keyword">function</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
        
        <span class="token keyword">var</span> object<span class="token operator">=</span><span class="token punctuation">{</span>
            <span class="token literal-property property">next</span><span class="token operator">:</span><span class="token number">0</span><span class="token punctuation">,</span>
            <span class="token function-variable function">stop</span><span class="token operator">:</span><span class="token keyword">function</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">{</span><span class="token punctuation">}</span>
        <span class="token punctuation">}</span>
        
        <span class="token keyword">return</span> <span class="token punctuation">{</span>
 			<span class="token function-variable function">next</span><span class="token operator">:</span> <span class="token keyword">function</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
 			<span class="token keyword">var</span> ret <span class="token operator">=</span> <span class="token function">cb</span><span class="token punctuation">(</span>object<span class="token punctuation">)</span><span class="token punctuation">;</span>
 			<span class="token keyword">if</span> <span class="token punctuation">(</span>ret <span class="token operator">===</span> <span class="token keyword">undefined</span><span class="token punctuation">)</span> <span class="token keyword">return</span> 			<span class="token punctuation">{</span> <span class="token literal-property property">value</span><span class="token operator">:</span> <span class="token keyword">undefined</span><span class="token punctuation">,</span> <span class="token literal-property property">done</span><span class="token operator">:</span> <span class="token boolean">true</span> <span class="token punctuation">}</span><span class="token punctuation">;</span>
 			<span class="token keyword">return</span> <span class="token punctuation">{</span>
 			<span class="token literal-property property">value</span><span class="token operator">:</span> ret<span class="token punctuation">,</span>
			 <span class="token literal-property property">done</span><span class="token operator">:</span> <span class="token boolean">false</span>
 			<span class="token punctuation">}</span><span class="token punctuation">;</span>
 		<span class="token punctuation">}</span>
 <span class="token punctuation">}</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span><span class="token punctuation">)</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br><span class="line-number">20</span><br></div></div><h3 id="q11">11. Promise</h3> <ul><li><p>Promise是ES6新增的语法，解决了回调地狱的问题</p></li> <li><p>可以把Promise看成一个状态机，初始pending状态，可以通过resolve和reject，将状态转变为resolved或者rejected状态，状态一旦改变就不能再次变化</p></li> <li><p>then函数会返回一个Promise实例，并且该返回值是一个新的实例而不是之前的实例，因为Promise规范除了pending状态，其他状态是不可以改变的，如果返回同一实例的话，多个then调用就失去意义了</p></li></ul> <h3 id="q13">13. ``==``和``===``区别，什么情况用``==``</h3> <blockquote><p>[]==![]</p></blockquote> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token operator">==</span><span class="token boolean">false</span>
<span class="token punctuation">[</span><span class="token punctuation">]</span>隐式类型转换，<span class="token operator">==</span><span class="token operator">&gt;</span><span class="token boolean">false</span>

<span class="token operator">==</span> 判断值类型是否相同

<span class="token boolean">false</span><span class="token operator">=</span><span class="token boolean">false</span>


<span class="token operator">==</span><span class="token operator">&gt;</span><span class="token boolean">true</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br></div></div><ul><li>===判断值类型和引用类型同时相同</li></ul> <h3 id="q15">15. 浏览器 Eventloop 和 Node 中的有什么区别</h3> <blockquote><p>js是非阻塞单线程语言，最初JS就是为了和浏览器交互诞生的，如果JS是门多线程的语言话，我们在多个线程中处理DOM就可能发生问题（一个线程中新加节点，另一个线程删除节点）</p></blockquote> <ul><li>JS在执行过程中会产生执行环境，这些执行环境会被顺序的加入到执行栈中。如果遇到异步的代码，会被挂起并加入到Task队列中，一旦执行栈被清空，Eventloop就会从Task队列拿出需要执行的代码放入执行栈中执行，所以本质上来说JS中的异步还是同步行为</li></ul> <div class="language-js line-numbers-mode"><pre class="language-js"><code>console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'script start'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token function">setTimeout</span><span class="token punctuation">(</span><span class="token keyword">function</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
 console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'setTimeout'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span><span class="token punctuation">,</span> <span class="token number">0</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'script end'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><ul><li><p>以上代码虽然 setTimeout 延时为 0 ，其实还是异步。这是因为 HTML5 标准规定这个
函数第⼆个参数不得⼩于 4 毫秒，不⾜会⾃动增加。所以 setTimeout 还是会在
script end 之后打印。</p></li> <li><p>不同的任务源会被分配到不同的Task队列中，任务源可以分为微任务和宏任务</p></li></ul> <div class="language-js line-numbers-mode"><pre class="language-js"><code>console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'script start'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token function">setTimeout</span><span class="token punctuation">(</span><span class="token keyword">function</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
 console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'setTimeout'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span><span class="token punctuation">,</span> <span class="token number">0</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">new</span> <span class="token class-name">Promise</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token parameter">resolve</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
 console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'Promise'</span><span class="token punctuation">)</span>
 <span class="token function">resolve</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">then</span><span class="token punctuation">(</span><span class="token keyword">function</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
 console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'promise1'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">then</span><span class="token punctuation">(</span><span class="token keyword">function</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
 console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'promise2'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'script end'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">// script start =&gt; Promise =&gt; script end =&gt; promise1 =&gt; promise2 =&gt; setTime</span>

</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br></div></div><ul><li><p>以上代码虽然 setTimeout 写在 Promise 之前，但是因为 Promise 属于微任务⽽
setTimeout 属于宏任务，所以会有以上的打印。</p></li> <li><p>微任务包括</p></li></ul> <div class="language-js line-numbers-mode"><pre class="language-js"><code>process<span class="token punctuation">.</span>nextTick ， promise ， Object<span class="token punctuation">.</span>observe ，
MutationObserver
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><ul><li>宏任务包括</li></ul> <div class="language-js line-numbers-mode"><pre class="language-js"><code>script ， setTimeout ， setInterval ， setImmediate ， <span class="token constant">I</span><span class="token operator">/</span><span class="token constant">O</span> ，
<span class="token constant">UI</span> renderin
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><blockquote><p>浏览器会先执⾏⼀个宏任务，接下来有异步代码的话就先执⾏
微任务</p></blockquote> <h4 id="一次完整的事件循环"><a href="#一次完整的事件循环" class="header-anchor">#</a> 一次完整的事件循环</h4> <ul><li>执行同步代码，这属于宏任务</li> <li>执行栈为空，查询是否有微任务</li> <li>执行所以微任务</li> <li>必要渲染UI</li> <li>开始下一轮Event loop</li></ul> <h3 id="q16">16.  setTimeout 倒计时误差
</h3>
&gt;JS 是单线程的，所以 setTimeout 的误差其实是⽆法被完全解决的，原因
&gt;有很多，可能是回调中的，有可能是浏览器中的各种事件导致。这也是为什么
&gt;⻚⾯开久了，定时器会不准的原因，当然我们可以通过⼀定的办法去减少这个
&gt;误差。</div></div>  <div class="page-edit"><div class="edit-link"><a href="https://github.com/linxin1123/vuepress-blog/edit/master/docs/08.面试/02.前端面试/02.进阶篇/01.JavaScript进阶(一).md" target="_blank" rel="noopener noreferrer">编辑</a> <span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></div> <!----> <div class="last-updated"><span class="prefix">上次更新:</span> <span class="time">2023/02/26, 19:02:51</span></div></div> <div class="page-nav-wapper"><div class="page-nav-centre-wrap"><a href="/pages/50d3e4/" class="page-nav-centre page-nav-centre-prev"><div class="tooltip">JavaScript(六)</div></a> <a href="/pages/63837c/" class="page-nav-centre page-nav-centre-next"><div class="tooltip">第一阶段-cnblog</div></a></div> <div class="page-nav"><p class="inner"><span class="prev">
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